The present invention relates to wide range viscosity multi-grade lubricants. This is a class of lubricants that, because of their wide viscosity range, permit use where the lubricant must maintain its effectiveness across a wide temperature range.
Lubricant viscosity is usually graded using SAE (Society of Automotive Engineers) designations. These are well defined in the industry. Depending on the final use, there are other standards which must also be met including wear properties and resistance to oxidation. Thus, for example, for a wide viscosity lubricant to be useful as a multigrade gear oil, it must not only maintain the appropriate viscosity, but must also pass a so-called MACK Standard Test 5GT73 "Transmission Test for Evaluation of Thermally Stable Gear Oil." This is, in essence, a test which requires survival of the lubricant when subjected to a predetermined number of "shifts" under predetermined conditions in a transmission for a Mack truck. The tests are available at independent laboratories and are industry standards for certain commercial purposes (especially gear box lubricants).
The uses of wide range viscosity multigrade lubricants are many. These include multigrade gear oil (SAE 80W-140) for use in gear boxes (final drives or axles of trucks or transmissions in a truck or heavy equipment) hydraulic oils, metal working fluids and possible engine oils for special purposes. In general, a wide range viscosity multigrade lubricant can allow equipment to be started under extreme low temperatures and be placed under load fairly quickly because the lubricant has low viscosity characteristics at low temperatures. Furthermore, because the lubricant has a wide range viscosity, it maintains effectiveness even at operating temperatures and under load for the equipment. Without the use of wide range multigrade viscosity lubricants, it may be necessary either to start, for example, a hydraulic pump and let it warm up several hours before it can be used under load, or to keep equipment operating at idle to avoid such a warm-up period. Otherwise, in cold weather, the lubricant will solidify or freeze and not be available to lubricate the equipment. Wide range lubricants prevent this freeze-up at low temperatures while providing adequate lubrication at higher operating temperatures.
As can readily be understood, wide viscosity lubricants can be very important under a wide range of actual operating conditions for many applications. It is known to formulate various lubricants to provide wide range viscosity characteristics in order that the temperature range of service for the lubricant can be extended. However, these formulations can be costly especially for widest range formulations to be used under extreme conditions.
It is known that the temperature range of service for gear oils and hydraulic oils can be extended by adding polymeric thickeners viscosity index improvers (VII's) and wax crystal modifiers (pour point depressants or "PPD's") to relatively nonviscous base fluids of both mineral oil and synthetic types. Common commercial polymer thickeners include low molecular weight polyalkyl methacrylates and polyisobutylenes (PIB) used in gear oils, predominately polyalkyl methacrylates with MW of 10,000-2,000,000 used in high viscosity index (VI) hydraulic oils, and a variety of thickeners including styrene isoprene block copolymers, olefin copolymers, and polyalkyl methacrylates for use in multigrade engine oil. Various PPD's are added to all these oils to improve low temperature pumpability. Alternative systems employ synthetic fluids such as polyalpha olefins (PAO's) and polyol esters to meet the industry's viscometric requirements, but at premiums in cost of 400% or more.
As discussed above, specific to gear oils is a Society of Automotive Engineers (SAE) rating system which defines the useful Operating temperature of the oil based on results obtained from specific American Society for Testing and Materials (ASTM) tests. The rating system imposes cold and hot temperature restraints. For example, a gear oil having an SAE grading of "140" must have a kinematic viscosity (as measured by ASTM D-445) of greater than 24 centistokes (cSt) at 100.degree. C. To obtain an "80W" rating it must have a viscosity (as measured by ASTM D-2983) of less than 150,000 centipoise (cP) at -26 C. A fluid which meets both constraints concurrently obtains a viscometric rating of 80W-140. Similarly, a fluid with a greater than 13.5 cSt kinematic viscosity at 100.degree. C. and a viscosity of less than 150,000 cP. at -40.degree. C. is rated a 75W-90 grade. This art has found that mineral oils alone or in combination with pour point depressants will not meet these requirements. Viscosity index improvers have been used in combination with pour point depressants to meet these requirements, but are inadequate for various reasons. Thus, although mineral oils have a cost advantage over synthetic-based lubricants, their useful temperature range is limited and until now could not be improved at low cost while maintaining a high quality lubricant.
To date, three major commercial multigrade gear oil systems are available.
(1) Very light (4-6 cSt at 100.degree. C.) mineral oils which have been treated with pour point depressants (PPD) to reach the 80W requirements. These oils are then thickened with large amounts (30% or more) of polyisobutylenes viscosity index improvers (PIB VII) to a 140 grade. The result is an 80W-140 gear oil.
However, gear oils using PIB viscosity index improvers have poor cold temperature performance as their primary disadvantage. PIB's barely meet the 80W viscometric requirements and are successful only when treating very light oils with large amounts of polymer and adding 2% or more supplemental pour point depressants and/or by adding an expensive "spike" of synthetic fluid of 5% or more. SAE 75W-90 grade oils cannot be produced with commercial PIB's and mineral oil because the cold temperature targets cannot be met.
(2) Mineral Oil Blends in the 6-12 cSt range at 100.degree. C. range thickened with polyalkyl methacrylates (PMA's) to a 140 grade, and 3-5 Cst at 100.degree. C. mineral oil blends thickened to a 90 grade. These blends solve the cold temperature problems but at the expense of often increased oxidation. Additionally, commercial PMA's used are in the 20,000-50,000 MW range and thus suffer from large viscosity losses of up to 50% in field performance. These loses push the fluid out of grade on the hot side, and result in lowered film strength and thus less wear protection. One alternative solution is to use low molecular weight PMA's with peak MW's below 10,000 which will shear less. These low MW PMA's are much less efficient thickeners requiring treat rates which are nearly doubled and making costs commercially unacceptable.
(3) Fully synthetic fluids such as blends of polyalkyl olefin (PAO's) and/or polyol esters. These blends provide the widest temperature range of operation and good oxidation performance. Their primary disadvantage is in their high cost of 3-5 times more than viscosity index improved mineral oils. Also some seal and additive compatibility problems can occur with these fluids.
In summary, the known use of these high molecular weight VI improvers, in the production of multigraded lubricants have some serious drawbacks:
a. They are susceptible to large permanent viscosity losses from mechanical shearing when exposed to the high shear rates and stresses encountered in gear boxes.
b. They struggle to meet or do not meet the cold temperature viscosity requirements.
c. They are often too costly to be employed.
d. They can be susceptible to oxidation, creating organic acids which can cause corrosion, wear, and/or formation of unwanted deposits.
e. They are susceptible to a high degree of temporary shear.
(Temporary shear viscosity loss is the result of the non-Newtonian viscometrics associated with solutions of high molecular weight polymers. It is caused by an alignment of the polymer chains with the shear field under high shear rates with a resultant decrease in viscosity. The decreased viscosity reduces the wear protection associated with viscous oils. Newtonian fluids maintains their viscosity regardless of shear rate.)
The use of low MW PMA's with light mineral oils has the disadvantage of requiring large treat rates to attain required results, so that costs are high. Similarly, costs are high with fully synthetic blends.
One solution to the problem of providing multiviscosity lubricants is described by Watts et al in U.S. Pat. No. 4,956,122 wherein certain combinations of fluids and additives are used to prepare multigraded lubricants which outperform prior art formulations and have none or a greatly decreased amount of the above listed deficiencies found in polymerically thickened oils. However, these fluids require expensive synthetic oil components. (See discussion (3) above.)
The present invention has an object is to provide a polymer system that can be added to mineral oil blends to produce wide range viscosity 80W-140 and SAE 75W-90 lubricants. This allows the use of relatively low cost mineral oils or "bright stock" in place of expensive polymers.
A further object is to provide wide range viscosity lubrication that also provides (1) the cold temperature performance of PMA's, (2) the oxidation and shear stability of PIB's, and (3) the low cost of VI improved mineral oils that meet industry requirements without expensive synthetics.